See Mercury this week!

On May 6th the very thin crescent of the Moon, low in the Western twilight after sunset, will be your guide to seeing the elusive planet Mercury!

Mercury is the innermost planet in the Solar System and orbits the Sun in a breathtaking 88 days - it loops around more than four times in the time it takes our planet to go around once. Yet Mercury is still damned hard to see. That's because it never strays far from the Sun in our sky, so most of the time it is low in twilight just after sunset and just after sunrise. Then there is the geometry of our own planet's orbit and axial tilt to take into account; the orbit of Mercury appears to be tilted at different angles at different times of the year, meaning that much of the time when Mercury is at its greatest distance from the Sun, it's still very close to the horizon - just well off to the left or right of the Sun. Except at certain times of the year. This week is one of those times!

Sky and Telescope has a nice article on this week's skyshow. And of course, Jack Horkheimer, Star Hustler (a.k.a. Jack Horkheimer, Star Gazer since the advent of search engines) has a show all about it. (See here for the script and here for the illustrations.)

The evening of Tuesday, May 6th is the big show, with the sliver-thin Moon passing just above and to the right of Mercury. But if you miss it that night, you still have a few more days to catch the speedy innermost planet before it passes out of the range of best visibility. By May 12 it will be at a higher position in the sky, but also much dimmer as it goes from a "waning gibbous" phase to a "last quarter" phase (see the diagram at Sky and Telescope.) And after that it begins a plunge into the evening twilight, and it will be a while before we can see Mercury again!


Also see this post on Phil Plait's Bad Astronomy:
http://www.badastronomy.com/bablog/2008/05/04/mercury-and-the-moon/